1. Field of the Invention
The present invention relates to a thermal head which is surface-modified to have a low surface tension without inhibiting the heat transferability thereof. More particularly, it relates to a thermal head capable of maintaining excellent perforatability over an extended period of time in plate-making of heat-sensitive stencil sheets.
2. Description of the Prior Art
There has been conventionally known a plate-making method using a thermal head as a plate-making method of heat-sensitive stencil sheets. In this plate-making method, a thermoplastic resin film side of a heat-sensitive stencil sheet is brought into contact with the thermal head to melt and perforate the thermoplastic resin film at portions corresponding to an image area of a manuscript by the application of heat of the thermal head.
However, when plate-making is continuously carried out with this method, the thermal melt of the film undesirably adheres to the surface of the thermal head, and hence the thermal perforatability of the thermal head is gradually reduced.
In general, thermal heads are classified into thin film type ones, thick film type ones, semiconductor type ones, and the like based on their respective structures. As shown in FIG. 1, a thin film type thermal head has a layered structure which is roughly divided into an insulation substrate 1, a heat-generating resistor 2 provided on the insulation substrate 1, a conductive layer 3 connected to the heat-generating resistor 2 for supplying electric power thereto, and a protective layer 4 covering both the heat-generating resistor 2 and the conductive layer 3. As shown in FIG. 2, a thick film type thermal head also has the similar layered structure which is roughly divided into an insulation substrate 1, a conductive layer 3 and a heat-generating resistor 2 provided on the insulation substrate 1, and a protective layer 4 covering the conductive layer 3 and the heat-generating resistor 2. Thus, the surface of the thermal head generally denotes the surface of the protective layer 4.
Materials used for the protective layer 4 are inorganic materials with relatively good heat transferability such as Ta.sub.2 O.sub.5, SiO.sub.2, SiON, and Si.sub.3 N.sub.3. However, such inorganic materials have high surface tension because of their high surface free energy, which makes the thermal melt of the film more likely to adhere to the surface of the thermal head.
In view of the foregoing circumstances, it has been proposed that a water- and oil-repellent and heat-resistant resin layer is further provided on the surface of the thermal head, that is, on the protective layer 4 to prevent the adhesion of the thermal melt of the film onto the surface (see Japanese Utility Model Publication No.Hei 4-7967, Japanese Patent Application Laid-Open Nos.Sho 60-2382, 60-178068, 62-48569, and the like). Such a resin layer is typically formed with fluororesin such as Teflon (tradename of Du Pont Corp.: polytetrafluoroethylene). Coating of the fluororesin on the surface of a thermal head generally requires the following procedure. First, a dispersion containing 50-60% solid polytetrafluoroethylene is prepared. Then, the dispersion is applied onto the surface of the thermal head, predried, and heated up to about 350.degree. C.
The fluororesin layer is excellent for ensuring the lower surface tension of the thermal head surface. However, since the treatment process (heat-treatment process) thereof applies heavy thermal load to the electronic components attached to the thermal head, it cannot be said to be a simple and suitable treatment method. Further, the fluororesin has also presented a problem in terms of its insufficient adhesion to the glass material of the protective layer.
Moreover, since the resin layer is a coating layer formed of a resin, it has a thickness of about 1 .mu.m even when thinly applied, and hence it inhibits efficient heat transfer from a heat-generating resistor to the surface. Further, there is also a limit to enhancement of the surface smoothness by making the film thickness of the resin layer uniform. Actually, the units of the resulting thickness and surface roughness are of the micron order.
Especially, when the plate-making of heat-sensitive stencil sheets is conducted with such a thermal head, the irregularities of the resin layer formed on the thermal head surface inhibit the adhesion between the thermal head and the heat-sensitive stencil sheet, thereby reducing the heat transferability. Consequently, uniform perforations of the heat-sensitive stencil sheet cannot be ensured.